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Table 1. Representative Rates at Which Heat and Moisture Are
Given Off by Human Beings in Different States of Activity.
Copyright © 2007, American Society of Heating, Refrigeration and
Air-Conditioning Engineers, It is presented for educational
purposes only. This table may not be copied and/or distributed
electronically or in paper from without permission of ASHRAE
At rest,
over 50% of the sensible heat transfer between the
occupant and the room is radiant based - and yet 99.99% of all
thermostats measure air temperature - go figure.
Visit our
radiant design and performance characteristics page.
Glossary of Terms
Bibliography / resources
For a full overview on building codes and
controlling radiant see the inaugural issue of
BetterBuildings
For conditioning of air with radiant based
HVAC see
Dedicated Outside Air Systems (DOAS)
"Due to the density of water,
more energy can be captured and channelled with water than air.
It is difficult to imagine how or why the body would use air to
move heat energy; the size of the respiratory system, the
diameter of veins and arteries, and the caloric intake require
to thermal condition would be as absurd as it would be
inefficient. It is hard, then, to imagine why we design and
condition buildings in this way."
Kiel Moe, Assistant
Professor, Architectural Technology,
Harvard University |
Definition of
Radiant Based HVAC Systems:
Copyright (c) 2004 Robert Bean, All rights reserved. For additional
support visit our
visitor services page.
Radiant based HVAC systems by its implied definition, integrates the
interior surface
temperatures of the building enclosure for the purposes of
evaluating the
mean radiant temperature for establishing
thermal comfort based on
human factor design.
Background: The control over surface
temperatures for thermal comfort is not new; to set the bookends as it were, are two quotes one from
1857 and another 2010 - both from internationally recognized authorities on health and
buildings advising us to focus on the enclosure to control
heat
transfer by radiation:
1857 - "the Commissioners of the General Board of
Health advocated as one of the requirements for comfort
that the
walls of a room be at least as high in temperature as the
general temperature of the room, while they included
cold walls
or floors amongst the conditions which make for discomfort.”1
2010 - National Building Code of Canada v2010:
Section A-5.3.1.2.(1) Use of Thermal Insulation or Mechanical
Systems for Environmental Control states, “In addition to
controlling condensation, interior surface temperatures must be
warm enough to avoid occupant discomfort due to excessive
heat
loss by radiation.”
Furthermore, recognized authorities in human
factor design and thermal comfort are in agreement with the UK's
Health and Safety Executive which states,
“The
most commonly used indicator of thermal comfort is air
temperature – it is easy to use and most people can relate to
it. But although it is an important indicator to take into
account, air temperature alone is neither a valid nor an
accurate indicator of thermal comfort or thermal stress.”
Introduction:
Many people assume wrongly that
Radiant Based
HVAC Systems are exclusively mechanical or electrical solutions like floor heating.
 However, 'radiant' control is ideally
first a building science solution.
This means the building enclosure should be the prime solution
for
controlling the radiant exchange between the occupant and the interior surfaces.
This is achieved with building efficiency.
The higher the
performance in the building enclosure the less
temperature difference between the person and the room mass the
more likely the conditions for occupants to perceive thermal comfort. This begins to occur in my experience when
loads are reduced with architectural systems to less than approximately 12
Btu/hr/sf.
Mechanical solutions,
like low temperature radiant heating and high temperature
radiant cooling can then be applied if necessary to compensate for the buildings short comings.
Therefore, when we use the term Radiant Based
HVAC Systems, please understand we are first and foremost talking about the body’s radiant heat transfer to and from the built environment
(see Table 1.) and controlling it with architecture then
with mechanical solutions.
As illustrated below, low performing
buildings using traditional heating systems need high fluid
temperatures in heating (and low temperatures in cooling) but
only a high performance building can use low temperatures in
heating and high temperatures in cooling (applies to all systems). In fact with high
performing buildings, using radiant cooling and heating,
conductive floors and close tube spacing, fluid and surface
temperatures operate close to the core body temperature and
enable
maximum efficiency from the heating and cooling plant.
click to enlarge
Such a small difference between the
fluid temperatures in the pipes and the temperature in
your blood has a major benefit in that it greatly enhance the efficiency of chillers, condensing boilers, solar and heat pumps. So you win when you improve the building and you win
when you use low temperature radiant heating and high
temperature radiant cooling.
Learn about the benefits of using a
dedicated outdoor air system (DOAS, i.e. HRV's/ERV's)
operating in parallel with radiant cooling and heating systems.
To see an example of a high performance building with radiant heating and HRV's for ventilation click here.
See overview of
housing performance categories in North America.
1. Source: Subjective Impressions of Freshness In
Relation To Environmental Conditions by T. Bedford, D.Sc., Ph.D.
and C. G. Warner, Ph.D., B.Sc., of the Industrial Health
Research Board and the London School of Hygiene and Tropical
Medicine, 1939
<http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2199495/>
accessed Oc. 17, 2011
If you wish to discuss this further please visit the
Online Help.
see also:
Radiant Based
HVAC – What does it mean to indoor air quality?
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